1. Field of the Invention
The present invention relates to an inner rotor type brushless motor which has, for example, a rotor and a stator.
2. Description of Related Art
Generally, an inner rotor type brushless motor has a stator core fitted into and fixed to a stator case and a rotor rotatably provided with respect to the stator core, and includes permanent magnets at an outer peripheral portion thereof. This stator core includes a plurality of teeth which project radially inward. A coil is wound around these teeth. As electric current flows into the coil, an attractive or repulsive force is generated between the rotor and the stator to rotate the rotor.
Among this kind of stator core, there is one which includes a plurality of commutating-pole teeth formed on the internal diameter side of a cylindrical yoke portion, and a split tooth including a projection fitted into the dovetail groove is mounted on a coil bobbin around which a coil is wound to form a split core, and is then fitted into the dovetail groove (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2006-296033). A set bolt hole is formed in a root portion of a commutating-pole tooth around which the coil is not wound, and a set bolt is inserted into the set bolt hole during assembly. Thereby, the space where no coil can be use effectively, and the size of brushless motor is miniaturized.
Meanwhile, in the above-mentioned brushless motor, when the output of the motor is improved, for example, a magnetic flux generated from the coil should be improved by increasing the number of turns of the coil. However, the space (slot) between the adjacent commutating-pole teeth is limited by keeping space for a set bolt which connects stator core with bracket and by securing a magnetic path width corresponding to the magnetic flux. Therefore, in order to increase the number of turns of the coil, enlargement of the brushless motor itself or the complication of the winding of the coil is inevitable.
To address the above problem, improving the occupancy of the coil by forming the cylindrical yoke portion such that the thickness thereof is thin and by forming the commutating-pole teeth such that the length thereof is elongated in constant width may be considered. However, in this case, there is a problem in that the magnetic path width corresponding to the magnetic flux generated from the coil can not be secured. Further, increasing the output of the motor by elongating the length of the motor itself may be considered, but the size of the set bolts which connect stator with bracket should be enlarged. That is, the size of the set bolt hole formed in the cylindrical yoke portion also should be enlarged. As a result, the thickness of the cylindrical yoke portion needs to be formed thickly, and the size of motor itself becomes large.
Therefore, the present invention has been made in view of the above-described situations, and the object of the present invention is to provide a brushless motor capable of increasing the cross-sectional area of the coil with minimizing the size of the brushless motor.